1. Field of the Invention
The present invention relates to a sheet-decorating injection molding method for forming a resin molding decorated with patterns and letters by bonding a decorative sheet to the surface of a resin molding in a mold when forming the resin molding by injection molding, and a sheet-decorating injection molding machine for carrying the sheet-decorating injection molding method.
2. Description of the Related Art
Various sheet-decorating injection molding methods which bond a decorative sheet to the surface of a resin molding in a process of molding the resin molding in a mold by injection molding have been proposed. Most of those previously proposed sheet-decorating injection molding methods carry out all or some of the following steps (a) to (i) in that order, in order of different sequence of those steps, or carry out a plurality of steps among those steps (a) to (i) simultaneously or in parallel.
Step a: A sheet feed process for feeding a decorative sheet onto the parting surface of a movable mold
Step b: A sheet fixing process for fixedly holding the decorative sheet on the parting surface of the movable mold
Step c: A sheet softening process for softening the decorative sheet by heating the same by a hot plate or the like
Step d: A sheet drawing process (sheet preforming process) for drawing the decorative sheet so that the decorative sheet extend along the inner surface of the female mold defining a cavity by vacuum forming or pneumatic forming (Preforming process)
Step e: A mold clamping process for moving the movable mold (usually, a female mold) toward the fixed mold to close the injection mold
Step f: An injection molding process for forming a molding by injecting a fluid resin (molten resin) through the fixed mold into the cavity defined by the movable mold and the fixed mold
Step g: A mold opening process for separating the movable mold from the fixed mold to open the injection mold
Step h: A trimming process for removing unnecessary portions of the decorative sheet from a portion of the same bonded to the resin molding
Step i: A molding removing process for removing a sheet-decorated molding from the injection mold
A plurality of processes among the foregoing processes may be carried out simultaneously. For example, the sheet fixing process of Step b and the mold clamping process of Step e can be achieved simultaneously by fixedly holding the decorative sheet between the movable mold and the fixed mold by the mold clamping process of Step e. The sheet drawing process of Step d and the injection molding process of Step f can be achieved simultaneously if the decorative sheet is drawn by the heat and pressure of the molten resin injected into the cavity in the injection molding process.
Either a laminated decorative sheet or a transfer decorative sheet is used selectively as the decorative sheet according to the type of the sheet-decorated molding. If a laminated decorative sheet is used, the laminated decorative sheet is bonded integrally to the surface of the resin molding in a decorative layer. If a transfer decorative sheet is used, only a transfer layer, i.e., a decorative layer, of the transfer decorative sheet is transferred to the resin molding to form a decorative layer on the resin molding and a base sheet of the transfer decorative sheet is separated from the transfer layer.
Usually, the sheet-decorating injection molding machine which carries out sheet-decorating injection molding carries out the sheet fixing process of Step b and the sheet softening process of Step c prior to the sheet drawing process of Step e. Therefore, the sheet-decorating injection molding machine is provided with a sheet clamping means for fixedly holding the decorative sheet on the parting surface of the movable mold, and a hot plate for heating and softening the decorative sheet fixedly held in place by the sheet clamping means, capable of being moved near to and separated from the decorative sheet.
The sheet clamping means often is a three-dimensional sheet damper such as proposed in JP-A No. 7-227877 when a molding to be produced has a highly irregular shape or an arched profile, such as the shape of a control panel for a rice cooker. This previously proposed three-dimensional sheet damper has a frame-shaped sheet clamping part for fixedly pressing a decorative sheet on the parting surface of a movable mold, and a sheet pressing part having two-dimensionally curved or protruded shape for pressing a portion of the decorative sheet into a recess formed in the movable mold.
Usually, the hot plate is such as using infrared rays for heating. Representative previously proposed hot plates include a flat hot plate disclosed in, for example, JP-B No. 63-6339 or JP-A No. 5-301250, a curved (or polygonal) hot plate proposed by the applicant of this patent application in JP-A No. 7-227827, and a flexible hot sheet employing a flexible surface heating element proposed by the applicant of this patent application in JP-A No. 8-39602. The flat hot plate, the curved (or polygonal) hot plate and the flexible hot sheet has the following disadvantages.
If a molding to be produced has a highly irregular shape or an arched profile, most part of a decorative sheet to be bonded to the irregular surface of the molding to be produced is pressed into a recess formed in a movable mold before heating and softening the decorative sheet by the three-dimensional sheet damper so that the decorative sheet is deformed so as to conform approximately to the surface of the recess corresponding to the surface of the mold to be produced. Therefore, if the flat hot plate of JP-A-No. 5-301250 is employed for heating the thus deformed decorative sheet, the distances between the heating surface of the flat hot plate and portions of the decorative sheet are distributed in a wide distance distribution range; that is, a portion of the decorative sheet near the parting surface of the movable mold is at a short distance from the heating surface of the flat hot plate, while a portion of the decorative sheet pressed deep into the recess by the three-dimensional sheet damper is at a considerably long distance from the heating surface of the flat hot plate. Therefore, the difference in distance from the heating surface of the flat hot plate between a portion nearest to the heating surface of the flat hot plate and a portion farthest from the same is very large. Consequently, portions of the decorative sheet are heated at different temperatures distributed in a wide temperature distribution range, respectively, so that the portions of the decorative sheet are drawn irregularly, a pattern formed on the decorative sheet is deformed, the pattern is dislocated from a correct position on the molding and, if things come to the worst, the decorative sheet is creased or broken.
If the curved (or polygonal) hot plate of JP-A-No. 7-227827 is used, the difference between the maximum and the minimum distance between the decorative sheet and the curved (or polygonal) hot plate can be reduced and hence portions of the decorative sheet are heated respectively at temperatures distributed in a temperature distribution range narrower than that of the temperatures of portions of the decorative sheet heated by the flat hot plate. Thus, the curved (or polygonal) hot plate of JP-A-No. 7-227827 is able to heat the decorative sheet more uniformly than the flat hot plate of JP-A No. 5-301250. However, the curved (or polygonal) hot plate needs high manufacturing cost because its heating surface having a three-dimensional shape requires a costly manufacturing process, has a weight far greater than that of the flat hot plate, requiring a large driving means including a hydraulic cylinder actuator of a large capacity or the like for holding and moving the curved (or polygonal) hot plate between a standby position and a sheet heating position, requiring an increased space for installation and increasing equipment cost, and is capable of being applied only to producing moldings of dimensions in a narrow range and having limited shapes because the shape of its heating surface must conform to the shape of the recess of the movable mold and cannot be altered and, basically, conforms to the shape of only one molding. Thus, the curved (or polygonal) hot plate must be manufactured specially for one molding, which increases initial cost.
The flexible hot sheet of JP-A No. 8-39602 is capable of avoiding the problems in JP-A No. 5-301250 and JP-A No. 7-227827 to some extent. However, the flexible hot sheet needs a complicated, large-scale driving means for holding, guiding and moving the flexible hot sheet between a standby position and a sheet heating position inside a three-dimensional sheet damper increasing equipment cost, is liable to fail to function properly because the flexible hot sheet provided with a flexible surface heating element is deformed for every injection molding cycle and the surface heating element is liable to break due to fatigue, and the flexible surface heating element cannot be bent in a curvature exceeding a limit curvature thereof to deform the flexible surface heating element in a shape conforming to the surface of a molding having a shape having irregularities of curvatures exceeding the limit curvature of the flexible surface heating element.
The present invention has been made in view of the foregoing problems in the prior art and it is therefore an object of the present invention to provide a sheet-decorating injection molding machine provided with an improved hot plate capable of heating a decorative sheet in a comparatively uniform temperature distribution as compared with the prior art flat hot plate, having a simple, lightweight construction as compared with the prior art curved (or polygonal) hot plate, less expensive than the prior art curved (or polygonal) hot plate, capable of being used for molding moldings of sizes in a relatively wide range and of a variety of shapes, less subject to failure than the prior art flexible hot sheet, and capable of being moved between a standby position and a sheet heating position by a relatively small, simple driving means.
Another object of the present invention is to provide a sheet-decorating injection molding method to be carried out by the foregoing sheet-decorating injection molding machine of the present invention.
According to one aspect of the present invention, a sheet-decorating injection molding machine is provided with a hot plate comprising a first heating plate and a second heating plate joined together in a V-shape with their corresponding edges abutting on each other and their heating surfaces facing outside.
The hot plate of the present invention may be of either a contact heating type using heat conduction for heating a decorative sheet or a noncontact heating type using radiation heat or dielectric heating for heating a decorative sheet. The hot plate of the present invention is able to heat a decorative sheet most effectively by a noncontact heating method using infrared radiation heat or the like. The hot plate of the present invention can be relatively easily constructed by joining together two conventional flat hot plate in a V-shape, or by bending a conventional flat hot plate in a V-shape and holding the hot plate by a suitable holding means. The heating surface of the hot plate need not necessarily be a combination of two flat surfaces; the heating surface may be formed in a polygonal profile as shown in FIG. 6(A) or in a curved profile as shown in FIG. 6(B) according to the shape of the outer surface of a molding to which a decorative sheet is to be bonded. The shape of the heating surface of such a hot plate resembles the shape of the outer surface of a molding to which a decorative sheet is to be bonded (the shape of the surface of a recess defining a cavity) and the shape of a sheet pressing portion of a three-dimensional sheet clamper. Therefore, the difference between the maximum and the minimum distance between the heating surface of the hot plate and portions of a surface to be heated of a decorative sheet is relatively small and hence the hot plate having such a heating surface having a curved or polygonal profile is able to heat the decorative sheet more uniformly than the hot plate having a V-shaped heating surface consisting of two flat surfaces.